The Vatican gave no immediate details about what was ailing the 77-year-old Argentine pontiff, inset, who has cancelled a number of engagements in the past few weeks due to minor health problems.

People had already gathered at Rome's Gemelli hospital to await the Pope, when the Vatican issued a statement saying Angelo Scola, Cardinal of Milan, would read a homily in his place.

Italian media said the Pope had postponed the appointment because he was "very tired". But Jesuit Fr Federico Lombardi, a Vatican spokesman, later said: "Because of a sudden ailment, the Holy Father will not go to the Gemelli for the announced visit,".

He declined to give more information about the Pope's condition.

Staff members, patients and their families were already gathered in the hospital lobby waiting for the Pope to arrive. Pediatric patients were seated in the lobby and a microphone for the 77-year-old Pope was ready.

Domenico Gianni, the head of Vatican security who often travels with the Pope, and other officers, as well as photographers from the Vatican newspaper also were waiting.

"The Pope stayed home," Lombardi said.

Earlier yesterday he had given his daily Mass at Santa Marta in the Vatican, and held a series of audiences.

The pontiff, who suffers from sciatica, or leg pain caused by a pinched nerve, cancelled a scheduled walk in a procession through the streets of Rome last week, a decision the Vatican said was made to conserve his strength for coming engagements.

He cancelled an engagement the week before due to a slight fever, but subsequently made a number of public appearances during which he seemed to be in good health.

The spokesman said the visit would be rescheduled.

At least four times in the past seven months, Pope Francis has cancelled appointments because he wasn't feeling well.

In December, the Pope postponed a meeting with Cardinal Angelo Scola of Milan and organisers of the World Expo that will be held there. In February, at the last minute, the Vatican said he was ill, so he cancelled a visit to Rome's major seminary.

His scheduled May 18 visit to Rome's Shrine of Our Lady of Divine Love was cancelled so the Pope could rest before going to the Holy Land on May 24-26. Most of his appointments on June 9 were cancelled because he wasn't feeling well.

In addition, because of the difficulty he has been having in walking long distances, Pope Francis chose not to walk in the mile-long Corpus Christi procession on June 19. Instead, he celebrated Mass at the Basilica of St John Lateran, then met the procession at the Basilica of St Mary Major.

Cardinal Scola presided at the Mass the Pope had been scheduled to celebrate at the Catholic University of the Sacred Heart and read the homily the Pope had prepared. In the text, Pope Francis had written that a Christian should see his or her life as an opportunity to witness God's love by humbly serving and caring for others.

The homily focused on Jesus' sacred heart as a sign and symbol of God's faithful, immeasurable love.

"God has set his heart on us, he has chosen us, and this bond is forever," the text said.

"We can experience and taste the tenderness of this love at every stage of our lives, in times of joy as well as sadness, in times of health as well as in sickness," Pope Francis had written.